200 LECTURE XVI. 



and in front of the optic nerves to the short seven-jointed antennae. 

 On each side also, but below the antennal nerves, arise the two nerves 

 (b) united together by an anastomosing branch which supply the palp- 

 less mandibles. 



The thick oesophageal chords (^) are continued from the pos- 

 terior and inferior angles of the brain ; and, though apparently simj^le^ 

 consist essentially of two chords, which become separate at the lower 

 part of the pharynx : the anterior chord girts the pharynx by a trans- 

 versely oval ring, formed by its confluence with its fellow ; the pos- 

 terior and normal columns converge, at an acute angle backwards, 

 blend together, and expand into the commencement of the abdominal 

 nervous trunk ; thus inclosing the oesophagus by a second and looser 

 collar. The closer anastomotic ring is analogous to that formed by 

 the transverse commissural band of the oesophageal chords in the 

 lobster and limulus ; and probably also to the anterior nervous ring 

 discovered by Lyonnet in the Cossus ligniperda. The stomato-gastric 

 nerves (/"), which arise from the posterior part of the brain, imme- 

 diately form a third slender ring (e), about the oesophagus, from the 

 middle of the upper part of which the trunk of the stomato-gastric sys- 

 tem is continued a short way back upon the stomach, when it divides ; 

 the two divisions diverge at an angle of 45°, bend abruptly back- 

 wards, and run parallel with each other along the dorso-lateral parts 

 of the wide and straight alimentary canal. 



Two large nerves (/i) are sent forwards from the beginning of the 

 thick suboesophageal or ventral chord (2, i), to supply the confluent 

 maxillae, which form the under lip : the nerves of the two single pairs 

 of feet, belonging to the thoracic segments, next arise, and afterwards 

 the more numerous minute nerves to the little feet, which, by their 

 articulation to the segments in double pairs, indicate such segments to 

 be severally a confluence of two. The simplicity of the abdominal 

 chords corresponds with the close approximation and great numbers 

 of the organs from which they receive impressions and to which they 

 transmit stimuli. The analogy of this exceptional condition of the 

 abdominal chord or nervous axis in the lulus to the dorsal spinal 

 chord of the Vertebrata, is as instructive as is that of the equally 

 exceptional ganglionic condition of the spinal chord in the Tetrodon 

 amongst fishes to the normal abdominal knotted chords in the Arti- 

 culata, in tracing their mutual relations to each other. 



The segments of the Polydesmus are relatively fewer and larger 

 than in the lulus, and their lateral margins are produced : each, how- 

 ever, with the exception of the first three, which answer to the thorax 

 in hexapod insects, supports two pairs of legs : but these are longer 

 than in the lulus. Accordingly we find the sub-abdominal nervous 



